Vaccines are the only feasible tool for effectively controlling the HIV pandemic. However, efforts to produce effective HIV vaccines have not yet been successful. This proposal focuses on an entirely new approach to designing HIV vaccines and will provide data directly applicable to the on-going efforts to develop safe, potent, and effective HIV vaccines. HIV envelope gp120 and gp41 are among the important antigens that must be targeted by the host immune system to fight HIV infection. While many studies have focused on antibodies and cytolytic CDST cells specific for these antigens, very little work has been done to study CD4 T helper responses which are needed to elicit and maintain effective antibody and CD8 T cell responses. In the majority of HIV infected patients, CD4 T helper responses to gp120 are undetectable. Gp120 also elicits weak lymphoproliferation in HIV-seronegative vaccine recipients. While many factors contribute to the poor CD4 T cell responses to gp120, two factors specifically interfere with class II MHC presentation of gp120 antigens to CD4 T cells: 1) antibodies to the CD4-binding site of gp120 which, upon binding to gp120, hinder gp120 proteolytic processing necessary for class II antigen presentation, and 2) the exceedingly rich glycosylation of gp120 which also deters the processing and presentation of this antigen. The goal of this application is to design gp120 immunogens that overcome the inherent properties of the native gp120 protein that suppress its processing and presentation. We propose to introduce mutations which alter the CD4- binding site or remove the N-glycosylation sites around the T helper epitope sites, or both (Aim 1). The mutated gp120 antigens will be expressed as soluble proteins and as membrane-bound proteins on HIV pseudovirions and analyzed for reactivity with a panel of human anti-gp120 monoclonal antibodies (Aim 2). The mutated antigens will be tested first in vitro for their capacity to stimulate gp120-specific CD4 T cell responses using established human T cell lines and PBMCs (Aim 3). Subsequently, we will use the gp120 plasmids and soluble proteins or pseudovirions for priming and boosting mice, and compare the capacity of the wild type and mutant constructs to elicit anti-gp120 CD4 T cell responses and the effector CDSand antibody responses (Aim 4). These studies will provide information about how to design more immunogenic forms of gp120, how to better stimulate gp120-specific CD4 T cell responses, and how to use the mutated gp120 constructs as HIV vaccines.